Fixes and improvements relating to scrolling

The changes here roll two issues into one change. With this commit:

- Scrolling up/down/etc using the keyboard now moves just one cell, rather
  than moving the number of cells specified by the scroll sensitivity that's
  intended for pointing devices. Textualize#1897
- Where appropriate the scrolling is done lazily; that is it is done after
  the next refresh, helping to ensure that the scroll will take into account
  any updates in the same parent call. Textualize#1774

CHANGELOG.md

@@ -13,6 +13,14 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `RadioButton` https://github.com/Textualize/textual/pull/1872
 - Added `RadioSet` https://github.com/Textualize/textual/pull/1872
 
+### Changed
+
+- Widget scrolling methods (such as `Widget.scroll_home` and `Widget.scroll_end`) now perform the scroll after the next refresh https://github.com/Textualize/textual/issues/1774
+
+### Fixed
+
+- Scrolling with cursor keys now moves just one cell https://github.com/Textualize/textual/issues/1897
+
 ### Fixed
 
 - Fix exceptions in watch methods being hidden on startup https://github.com/Textualize/textual/issues/1886